This study continues quantitative and qualitative investigations into the nature of energy metabolism in the fetus. A chronic fetal lamb preparation, and a chronic subhuman primate model (the fetal baboon) are being used to extend previous studies of quantitative energy metabolism. Substrate uptake and production by the ewes and by specific tissues of the fetus is being evaluated by utilizing appropriately placed arterial and venous catheters for sampling of arteriovenous metabolite differences, antipyrine infusions, and the radioactive microsphere technique for measuring fetal blood flow and specific organ blood flows. The effect of fasting and of maternal hypoglycemia upon fetal metabolism are currently under investigation.